Light influencing displays are well known in the prior art. They include liquid crystal, electroluminescent, and electrochromic displays.
It is common in such displays for there to be a plurality of individually addressable picture elements, called pixels, each having has a separate pair of opposing pixel electrodes, with light influencing material between them. Voltages are selectively applied to the electrode pairs, causing the light influencing material between them to emit light or change optical density. Commonly these opposing pixel electrodes are located on opposing substrates, such as opposing glass plates, with the light influencing material, such as liquid crystal material, located between the substrates.
In relatively small arrays of pixels, such as those found in many digital watches it is common to have all the pixel electrodes on one substrate connected in common and to have a separate address line for selectively applying voltages to each of the pixel electrodes on the second substrate, thus enabling each pixel to be individually selected . However, in large arrays, such as large x-y arrays, it proves impractical to have a separate address line connected to each pixel because too much room would be required by so many lines. Thus, in displays with many pixels it is common to use x-y matrix addressing, in which all the bottom pixel electrodes of a given row are connected to an address line associated with that row and all the top pixel electrodes of a given column are connected to an address line associated with that column. This enables each pixel to be addressed by selecting its associated x and y address lines.
In relatively small x-y arrays each of the x and y lines is usually directly connected, without intervening electronic devices, to the bottom or top pixel electrodes in its associated row or column. Usually the pixels of such an array are driven sequentially in a scanning process in which each row is selected successively, and, during the selection of each row, each column is selected successively, causing each pixels of each row to be scanned in sequential order. During the time allotted for the selection of each pixel the circuitry decides whether that pixel is to be turned on or off by controlling the amplitude of the voltage applied across the pixel. In parallel scanned arrays each of the rows is selected in succession, and during the selection of each row all of the column lines are driven in parallel with individually selected voltages, turning each pixel in the row to a desired state, either "on" or "off". Such multiplexing scanning schemes in which the pixels are driven directly by their x and y address lines without intervening electronic devices work well for relatively small arrays. But as array size grows, the amount of time for the application of a voltage to a given pixel, or row of pixels, decreases. As a result, the percentage of time during each scanning cycle that a given pixel has a voltage applied across it decreases, thereby decreasing the average contrast between its "on" state and its "off" state. For this reason large multiplexed displays tend to produce images with poor contrast, making such displays difficult to read.
One way of overcoming this limitation to use active display matrixes instead of multiplexed displays. In active display matrixes each of the pixels has associated with it a non-linear electronic devices which acts as a switch. This switch enables current to flow to or from the pixel during the brief period when the pixel is selected, enabling it to rapidly change state from "off" to "on", or vice versa, but it tends to prevent current flow to or from the pixel when it is not selected, so the charge placed on the pixel during its brief selection period is substantially maintained during the rest of the scanning cycle. As a result, the pixel tends to maintain its "on" or its "off" state during the entire scanning cycle, greatly increasing the contrast and readability of the display.
One type of electronic device commonly used with the pixels of active display matrixes is the three terminal control device, such as the transistor. A three terminal control device is one that has two current path electrodes and a control electrode, with the effective resistance between the two current path electrodes being controlled as a function of a signal supplied to the control electrode. For example, in the thin film field effect transistors often used in active matrix displays, the voltage applied to the gate electrode controls the amount of current which flows between the source and the drain of the transistor.
The three terminal devices are usually arranged on one substrate of the display, with one of their current path electrodes connected to an electrode of their associated pixel. In the prior art, crossing x and y lines are also placed on the substrate to enable each three terminal device and its associated pixel to be individually addressed. One of the sets of the address lines, for example, the x lines, are connected to the control electrodes of the three terminal devices, with each x line being connected to each of the control electrodes in its associated row of pixels. Correspondingly, the other set of address lines, the y lines, are connected to one of the current path electrodes of the control devices, with each y line being connected to a current path electrode associated with each pixel in its column. All the pixel electrodes on the display's other substrate are usually connected to a common voltage, such as ground. A given pixel is selected by applying a proper voltage between the x line connected to the control electrode of its three terminal device and the y line connected to the current path electrode of its three terminal device. For example, when the three terminal devices are thin film field effect transistors, the source of each transistor is connected to a y line and the gate of each transistor is connected to an x line. In such a structure, a selected transistor is turned on or off by providing the desired voltages between the x line connected to its gate and the y line connected to its source.